On this diagram, I want to point out the three different regions of the human respiratory system.
First, there is the nasal-pharangeal-laryngeal region, which encompasses the nasal cavity and the oral cavity just beyond nose and mouth,
the pharynx in the back of the throat where the two cavities come together, and the larynx (the voice box) just beyond the epiglottis,
the flap that separates the respiratory system from the digestive system.
Next is the tracheobronchial region, which encompasses the windpipe or trachea, the branching of the trachea into the two lobes of
the lungs, and the additional branching of the airways that is similar to the branches of a tree.
Finally, the air sacs at the very end of the branches of the tracheobronchial region,
shown in the inset, are the alveoli, or the alveolar region of the lung.
This is the part of the lung where gas exchange occurs, oxygen for carbon dioxide.
Because these regions of the lung have different dimensions, different processes govern the movement of particles in these regions,
and different sizes of particles may reach and deposit in the three regions, causing different health effects.
Due to the different dimensions of the various regions of the lung and the potential for
exposure to many different sizes of airborne particles, standard-setting organizations
have developed size-selective criteria for sampling aerosol particles that vary by the region of the lung.
The criteria were established by consensus among the American Conference of Governmental Industrial Hygienists,
the International Organization for Standardization, and the European Committee for Standardization.
Many samplers for airborne particles are designed according to these criteria to exclude
particles that do not reach the various regions of the respiratory system.
The inhalation criterion defines the fraction of particles at each size that can be breathed into the nose or mouth.
This fraction is referred to as inhalable particles.
The thoracic criterion, a subset of the inhalation criterion, defines the fraction of particles of each
size that can reach the tracheobronchial region of the lung and beyond. Then, the respirable criterion, a subset of the thoracic criterion,
defines the fraction of particles that can penetrate to the deepest parts of the lungs, the alveolar region.
This fraction is referred to as respirable particles.
So, the inhalable criterion includes those particles that can penetrate to all parts of the lung, the thoracic criterion includes particles
that can penetrate to the tracheobronchial and alveolar regions, and the respirable criterion
includes only those particles that can penetrate to the alveolar region.
This diagram shows the inhalable sampling criterion graphically.
The criterion is an equation for the fraction of particles at each size that can be inhaled.
Size, on the horizontal axis, is defined as the aerodynamic diameter, which is the diameter of a spherical particle with the density of water
that settles due to gravity at the same velocity as the particle you're interested in.
The aerodynamic diameter is shown on a logarithmic scale, which means that each order of magnitude is evenly spaced.
The fraction, from 0 to 1, is represented on the vertical axis on a linear scale.
The blue area represents the particles that are included in the inhalable sampling criterion.
The graph shows that about 50% of 100- micrometer diameter particles can be inhaled through the nose or mouth.
Almost all particles 1 micrometer and smaller can be inhaled.
In this graph, we superimpose the thoracic criterion in green on top of the inhalable criterion.
No particles larger than about 30 micrometers in aerodynamic diameter can reach the tracheobronchial region because they
can not enter the respiratory system or they do not make it past the nasal-pharyngeal-laryngeal region.
On the other hand, about half of the particles 10 micrometers in diameter and almost all particles
1 micrometer and smaller can reach the tracheobronchial region.
The remainder of the inhalable criterion in blue represents those particles that can be
inhaled but can not make it to the tracheobronchial region.
Here, we have the respirable criterion in red. No particles larger than 10 micrometers can reach
the alveolar region because they either can not be inhaled or they do not pass through the upper regions of the respiratory system.
About half of particles 4 micrometers in diameter can penetrate into the deepest part of the lung.
Most particles 1 micrometer in diameter and smaller can penetrate into the alveolar region.
As before, the blue area on the graph represents those particles that can be inhaled but can not make it to the tracheobronchial region.
The green area represents the particles that can pass to the tracheobronchial region but not into the alveolar region.
These criteria represent the particles that are in the air that enters each region of the respiratory system.
Put another way, these sampling criteria can be used to measure the potential dose for particles in each part of the lung.
The criteria do not represent the particles that deposit in the respiratory system, the applied
dose, because many of the particles will leave the respiratory system with exhaled air.
These sampling criteria are very useful to occupational hygienists, but as the Nobel Prize-
winning physicist Richard Feynman put it when talking about the possibility of building structures at the atomic and nanometer scale,
"There’s plenty of room at the bottom".
How do we think about and categorize particles even smaller than those considered in the
sampling criteria, including those at the nanometer scale?
In 1998, the aerosol scientist Othmar Preining defined fine particles as those that are 1
micrometer in aerodynamic diameter and smaller.
This is a fairly common definition although ASTM, formerly the American Society for
Testing and Materials, defines fine particles as those 2.5 micrometers and smaller.
This figure shows just the equations for the three sampling criteria: the inhalable criterion in blue,
the thoracic criterion in green, and the respirable criterion in red.
When we look at Preining's fine particle definition against the size-selective sampling
criteria, we see that fine particles in yellow are among the respirable particles that
can penetrate to all regions of the respiratory system.
However, fine particles exclude the many particles larger than 1 micrometer that can
penetrate to all parts of the respiratory system, including the alveolar region of the lung.
Preining defined ultrafine particles as being those smaller than 0.1 micrometer, or 100 nanometers, in aerodynamic diameter.
This is also a fairly common definition.
When we include the ultrafine definition in purple on the same graph as the fine particle and sampling criteria definitions,
we see that we are now talking about "stuff", using Smalley's word, that are within two orders of magnitude of the nanometer scale.
We've talked about a bunch of different size- based definitions of particles. So, where do nanoparticles fit in?
There are many definitions we should think about related to nanoparticles.